application noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.deAnalyzing Solar Cell Material Properties with Confocal Raman Imaging
Energy generation using photovoltaic devices
is regarded as an important component inovercoming future energy shortages. This isreflected in a dramatic increase inphotovoltaic production and demand. In theresearch and development of photovoltaic
devices, the primary goals are to increase theconversion efficiency of the solar cells or toimprove the production process. For thesestudies, a detailed knowledge of themicro- and nanostructures along with thechemical properties is essential for furtherimprovements. A valuable tool for theseinvestigations is Confocal Raman Imaging,as it not only reveals optical information but
also information regarding the 3D
distribution of the chemical compounds,
crystallinity and material stress. The
Fraunhofer-Institute for Solar Energy Systems(ISE), one of the leading institutes in the
research and development of solar cellsystems, employs the alpha500 Confocal
Raman AFM Microscope for the analysis ofphotovoltaic (PV) cells and polymeric module
components. In a joint research project, WITec
Side viewcontacts

and the ISE (Durability Analysis andEnvironmental Engineering Group)
investigated several solar cell samples
relevant to the solar cell industry using
Confocal Raman Microscopy. This article
describes the application of Confocal Raman
Imaging for the analysis of stress fields
around laser-drilled holes on a Si solar cell aswell as the investigation of concentrationdependent properties of a dye solar cell.
Stress Fields surrounding a Laser-drilled Holeon a Si Solar Cell DeviceTypical Si-based solar cells are principally built
as shown in Fig. 1 (sample courtesy of
Fraunhofer-Institute for Solar Energy Systems
ISE). The contacts for discharging are locatedon top of the p- and n-doped material.
It is now generally considered better to avoid
placing the contacts on the top surface, as

they block some of the sunlight and thus lower
the efficiency. Therefore holes need to bedrilled through the solar cells in order to allow
the contacts to be located on the bottom of
the solar cells. These holes are typically drilledwith lasers. The drilling process however caninduce strain in the Si around the holes, which
in turn lowers the efficiency of the solar cells.
In this study two ways of minimizing the stressare investigated:
1. Minimizing the laser power for drilling usinga) 100% drilling power with 9 repetitionsb) 20% drilling power with 20 repetitions
2. Etching the wafer after the drilling process:a) no etchingb) 1 Minute etching with KOHc) 2 Minutes etching with KOHd) 4 Minutes etching with KOHe) 6 Minutes etching with KOH

Top view

n-doped Materialp-doped Material

Fig. 1: Typical set-up of a solar cell.

Fig. 3: Video Image of a typical scan area (red box) on the SI.
The hole is visible on the bottom right corner.
Fig. 2: Splash of Si onto the surface out of the hole (bottom right). Plotted is the integrated intensity of the 1st order Si peak.

application noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.de

In order to evaluate the strain induced by thedrilling on the Si around the holes, Confocal
Raman Imaging was used. These
measurements were performed with a WITecalpha700 system using a 532nm excitation
laser (approximately 17mW on the sample), a100x air objective (NA 0.9).
The scattered light was directed to thespectrometer using a 50µm core diameter
multimode fiber which also acts as thepinhole for confocal detection. The
spectrometer was a WITec UHTS 300
equipped with a back-illuminated CCD camera
and 1800 grooves/mm grating. The
integration time for each spectrum was 23ms
and 150x150 pixels (=22500 spectra) over anarea of 35x35µm. The total acquisition time
per image was less than 10 Minutes.
Some of the holes showed splashes of Si on
the surface of the wafer which resulted in ahigher 1st order Si peak on these structures

as shown in the figure 2, where the integrated
intensity of the Si peak is evaluated at eachpixel and displayed as an image. However,these areas were avoided for the strain
measurements, while holes with relatively
smooth looking surroundings (see figure 3 forexample) were selected.
For each image the 1st order Si peak was fittedusing a Lorentzian curve and the exact peak
position was then evaluated. This could also
be shown as an image and an example isshown in figure 4. From these images, crosssections as shown in Fig. 4 were extracted
and plotted as relative peak positions (inrel.1/cm) vs. distance (note that the starting
point relative to the edge of the hole varies
for each line). Figures 5 and 6 show the resultsof these evaluations. Fig. 5 shows the results
for the samples with full drilling power andthe different etching processes applied to thesamples are indicated by the colors [noetching (red), 1 min etching(blue), 2 minetching (green), 4 min etching(black), 6 minetching (magenta)]. Fig. 6 uses the same color
coding, but shows the results for the cases
drilled with only 20% power. For most graphs
a slight decrease in the peak position can beseen starting from the left side of the graphand moving towards the position of the drilledhole. These are presumably long-range stressfields in the Si. Close to the edge of the hole,
the stress in the Si increases and changes
dramatically and then inside the hole, noproper Si signal could be detected.
From Fig. 5 it can clearly be seen that thesample which had not been etched as well as
the sample which had been etched for only 1
minute show a clear drop of the peak position
before reaching the edge of the hole. Thisdrop is eliminated for the other cases. FromFig. 6 the same conclusion as above can bedrawn. Additionally, the graphs seem toindicate that the soft drilling process reduces
the far reaching stress field (the slope of thelines). Both graphs indicate that an etchingprocess of 2 minutes is sufficient.

Fig. 4: A typical stress map ofthe Si (Position of the 1st orderSi line) from the outside to thehole (bottom right) Along theindicated cross section (grayline), the radial change of thestress field was evaluated.

Fig. 5: The cross sections of the various samples drilled with 100% laser power: no etching (red), 1min etching (blue), 2 min etching (green), 4 min etching (black), 6 min etching (magenta).

Fig. 6: The cross sections of the various samples drilled with 20% laser power: no etching (red),
1 min etching (blue), 2 min etching (green), 4 min etching (black), 6 min etching (magenta).

application noteLise-Meitner-Straße 6, D-89081 Ulm, Germany
Tel. +49 (0) 731 140 700, Fax. +49 (0) 731 140 70200
www.witec.de, info@witec.de

Dye Solar Cells

no TiO2 was applied and therefore no dye waspresent.

A test array for the observation of the distributionof the dye (attached to the TiO2) and the evaluationof the iodide in dye base solar cells was preparedas shown in Fig. 7 (sample courtesy of FraunhoferInstitute for Solar Energy Systems ISE). The iodideconcentration varied in the samples from 0% to600% of the concentration typically used in dye
bases solar cells.
The investigated cells had the following structure:
Glass with TCO (Transparent Conductive Oxide,3mm total thickness)
Inner area (total thickness ca 30-35µm):- TiO2 nano-porous area (ca. 10µm)- Monolayer dye Electrolyte consisting of:*ACN (acetonitril)*Ionic liquid (contains single iodide ions)*Iodide (varying in concentration and forming triiodide-ions)- Pt-layer as the catalyst
In the transparent areas of the cells (see figure 7)

As a first step, single spectra (10 accumulations
with 2s integration time) were taken from theareas without dye (through the 3mm glass) inorder to evaluate the change in the spectra
subjected to the iodide concentration changes(Fig. 8). It can clearly be seen that the peakassociated with iodide (located near 110 rel.1/cm)
is rising with the iodide concentration. However,at 0% there is still a significant peak present. Thisis due to the ionic liquid which itself contains iodideions.
In a second step, the border area between the dye
containing area and the transparent area wasimaged (again through the 3mm glass). The image
was acquired using a 50x objective (NA 0.5) with
80x80 pixels (=6400 spectra) and a scan range of
50 x 50 µm. For excitation, a 532 nm NdYag laser
was used. Acquisition time for one spectrum was
213 ms. The resulting spectra are shown in Fig. 9and also a zoomed view of the finger-print area inFig. 10.
Fig.11 shows the corresponding confocal Raman
image with the colors indicating where eachspectrum can primarily be found. Note that violet
areas indicate that a mixture of the blue and red
spectra can be found at these areas. The image incombination with the spectra indicate that theionic liquid plus the ACN (green spectrum) can befound in the green area. The area filled as instandard dye solar cells (darker areas in figure 7
and violet area in figure 11) shows a significantly
lower signal for the green spectrum but a strong
signal of a mixture of the red and blue spectra. The
blue spectrum shows the characteristic peaks ofthe dye near 1500 rel.1/cm, the intense peak near155 rel.1/cm and characteristiv peak at about 165
rel.1/cm. It is interesting that this additional
spectrum and thus also the material from which
this originates seems to be present without thedye at the transition area between the green andthe violet area.

Fig. 7: Image of the dye solar
cell samples. The
concentration of the iodinechanges from left to right as
600%, 200%, 100%, 50%, 0%.
In the clear parts on top of
the cells no dye was present.

Fig. 9: Typical Raman Spectra found in the border regionof the dye solar cell.

Fig. 8: The change in the spectra as a function of the iodine concentration:
red = 0%; turquoise = 50%; black = 100%; green = 200%; blue = 600%.

Fig. 10: Zoom of the spectra as seen in Fig. 9.

Fig. 11: Raman Image of the the border region of the dyesolar cell.